Lithium, the lightest alkali metal, has the lowest electro negativity. Its gravimetric electrochemical equivalence of 3.86 Ah/g is the highest among all metallic anode materials. Coupling a lithium anode with an air (oxygen) cathode can provide for very light, high energy density batteries. However, a significant hurdle to the commercialization of this technology is the difficulty in preventing dendridic lithium growth inside the cells of the battery during cycling.
During discharging the lithium in a lithium-air battery migrates from the anode to the cathode. Upon charging, the lithium is transferred back to the anode, however, the re-formation of the lithium at the anode is not uniform. During cycling, dendrimers of lithium form as radiating filaments from the lithium anode, eventually penetrating through the porous separator between the anode and cathode, and eventually contacting the cathode. The result is a shorting of the cell. This leads to eventual cell failure and potential violent reactions with liquid electrolytes. Consumption of liquid electrolyte via reactions with high surface area dendritic lithium during cycling is also a source of capacity fade in lithium air cells.
In lithium-air cells, the protection of the lithium anode from the air and moisture is of great concern. As noted, the cells employ a separator between the lithium anode and the air cathode, where the separator allows for lithium ion migration while preventing or minimizing oxygen and water incursion to the cell. One common material for preparation of the separator for lithium-air cells is known as a lithium super-ionic conductor (LiSICON) film. LiSICON (Li2O—Al2O3—SiO2—P2O5—TiO2GeO2; or generally (Li1+x+yAlx(Ti,Ge)2−xSiyP3−yO12) exhibits a lithium ion conductivity of 1×10−4 S/cm at 25° C. In the LiSICON, x may from 0 to 0.4, inclusive, and y may be greater than 0 and less than or equal to 0.6. While LiSICON allows for high lithium ion conductivity, it does not prevent dendritic lithium formation upon charging of the cell, and it is fragile and expensive material.